1. Field of the Invention
This invention relates to a liquid silicone rubber composition having improved flow, curing characteristics, and viscosity stability during shelf storage, and suitable for injection molding and a method for preparing the same.
2. Prior Art
Silicone polymers loaded with large amounts of reinforcing silica fillers are known to be improved in physical properties including tensile strength, tear strength, elongation, and heat resistance. In order to load silicone polymers with large amounts of silica, noticeable amounts of wetters must be added to improve the dispersibility of silica.
In the prior art, .alpha.,.omega.-siloxane diols of the formula: HO[(CH.sub.3).sub.2 SiO].sub.y H wherein y is 10 to 20, that is, having a degree of polymerization of 10 to 20 are widely used as the wetter. Other useful wetters are .alpha.-alkoxy-.omega.-siloxanols of the formula: R'O[(CH.sub.3).sub.2 SiO].sub.x H wherein R' is a short chain alkyl group and x is 3 to 5 as disclosed in U.S. Pat. No. 3,799,962. Further a mixture of hexamethyltrisiloxane diol and methoxyhexamethyltrisiloxanol is proposed as an anti-structure agent as disclosed in U.S. Pat. No. 3,925,285. Although the use of such hydroxyl-terminated silicone fluids as the wetter enables to blend large amounts of silica filler in silicone polymers, the resulting silicone polymer compositions are thixotropic, high viscosity compositions and thus unsuitable for injection molding because of the lack of flow.
Besides the above-mentioned silicone fluids, hexamethyldisilazane is also known as a wetter for liquid silicone rubber compositions. Although liquid silicone rubber compositions obtained using hexamethyldisilazane as the wetter are well flowing and less viscous, amines can be produced as by-products during blending of hexamethyldisilazane and they adversely affect curing behavior and cause thickening during shelf storage.